I have not bought any yet but I did a thorough check for what was available that covered the residence requirements and I decided that for price the cheapest at €50/month or €570 for an annual policy were these guys based in barcelona: https://www.fiatc.es/

I have not bought from them but rang them up and they said I needed to send them a current NIE certificate in order to receive a insurance certificate as proof for the residence application, and they seemed ok. They are an established company in Spain that has been around since 1930 apparently according to wikipedia https://es.wikipedia.org/wiki/FIATC